Liquid crystalline materials exhibiting the cholesteric mesophase have been proposed for use in a variety of optical device applications, for example, notch filters, circular polarizing filters, selective reflectors, beam splitters, and beam apodizers. U.S. Pat. No. 3,711,181, for example, discloses an optical apparatus for modulating circular-polarized light that contains optically negative liquid crystal films.
Polymeric liquid crystalline compositions have been proposed as potentially useful for the aforementioned applications. U.S. Pat. No. 4,293,435 discloses a liquid-crystalline cholesteric polymer phase that consists essentially of a copolymer of particular nematogenic and chiral acrylic ester monomers. In U.S. Pat. No. 4,410,570 is disclosed a liquid crystalline phase that contains a cyclic organopolysiloxane to which is chemically bonded at least one mesogenic group. Thermotropic cholesteric liquid crystalline glutamate copolymers consisting of chiral glutamate ester repeating units are disclosed in U.S. Pat. No. 4,743,675.
Besides polymeric liquid crystalline compositions, there has been an interest in discovering glass-forming liquid crystalline compounds of low molar mass. For example, Attard et al., Liquid Crystals, 1992, Vol. 11, No. 5, pp. 779-784 describes liquid crystalline compounds in which rod-shaped mesogenic groups are attached to a benzene-1,3,5-tricarbonyl nucleus. "Siamese-twin" molecules in which two rod-shaped mesogenic moieties are linked by a sulfonyl radical are described in Dehne and Roger, Liquid Crystals, 1989, Vol. 6, No. 1, pp. 47-62.
Attard and Imrie, Liquid Crystals, 1992, Vol. 11, No. 5, pp. 785-789, describes symmetrical liquid crystalline compounds containing a central alkylene chain and a 1-aminopyrene moiety at either end. Liquid crystalline "starburst" molecules, 1,3,5-tris 4-(diphenylamino)phenyl!benzene and its methylsubstituted derivatives, are described in Inada and Shirota, J. Mater Chem., 1993, Vol. 3(3), pp 319-320.
Kreuzer et al., Mol. Cryst. Liq. Cryst., 1991, Vol. 199, pp. 345-378, describes cyclic organopolysiloxane compounds containing mesogenic substituents. Liquid crystalline cyclic organopolysiloxanes containing cholesterogenic or chiral substituents are disclosed in U.S. Pat. No. 4,410,570 to Kreuzer and Gawhary. Freidzon et al., Polymer Preprints, 1993, Vol. 34(1), pp. 146-147, describes liquid crystalline cyclophosphazene compounds with cholesteryl-substituted mesogenic groups.
Another type of liquid crystalline composition of low molar mass is characterized by inclusion of mesogenic units as part of a macrocycle, as described, for example, in Percec et al., Macromolecules, 1992, Vol. 25, pp. 3851-3861.
Wedler et al., J. Mater. Chem., 1991, Vol. 1(3), pp. 347-356, notes that prevention of crystallization in liquid crystalline phases of pure compounds of low molar mass is a serious practical problem and proposes the use of mixtures of compounds. Wedler et al., J. Mater. Chem., 1992, Vol. 2(11), pp. 1195-1204, describes mixtures containing Siamese-twin and naphthyl ester mesogenic compounds. Mixtures of naphthyl esters to form liquid crystalline glassy compositions are also described in Schafer et al., Mol. Cryst. Liq. Cryst., 1990, Vol. 191, pp. 269-276. Eichler et al., Mol. Cryst. Liq. Cryst., 1992, Vol. 223, pp. 159-168, describes the use of mixtures of liquid crystalline compounds containing a cholesteric addendum for the formation of erasable holographic gratings.
Many applications of chiral liquid crystalline materials in optical devices require compositions capable of forming both right- and left-handed helical structures. When a film of such a composition is applied to a substrate, the helical structures must be capable of forming and maintaining the Grandjean texture, in which the helical axis is perpendicular to the substrate surface, to enable the selective reflection of circular-polarized light. An enantiomeric chiral pair of liquid crystalline compositions, whose individual structures are characterized as a right-handed and a left-handed helix, are thus capable of selectively reflecting right-handed and left-handed circular-polarized light, respectively. Especially useful for optical information storage applications are chiral nematic liquid crystalline compositions which form clear, transparent films that absorb no light in the visible region but do selectively reflect visible and near-infrared circular-polarized light.
Although thermotropic liquid crystalline polymers exhibiting stable mesophases at ambient temperature are well known, it is exceedingly difficult to achieve simultaneous mesophase formation and vitrification characteristics at temperatures above the ambient with compounds of low molar mass. A further challenge is presented by the objective of discovering chiral liquid crystalline materials of low molar mass that have the capability of forming both right-handed and left-handed helical structures. In addition, these materials must exhibit solubility characteristics which enable their processing into clear glassy thin films for use as optical devices. All of these requirements are met by chiral nematic liquid crystalline compositions with low molar mass of the present invention.